Such modulation schemes are now widely used in standards as a means to provide high data rates for communication systems including wireless local area networks (WLANs): ‘IEEE 802.11a’ in USA and ‘HIPERLAN/2’ in Europe, ADSL (Asynchromous Digital Subscriber Line) over twisted pairs and ‘HomePLUG’ on powerlines.
For the next decade, the challenge is to deliver an increased data rate coping with the requirements of multimedia broadband transmissions. None of the existing standards will be able to meet these requirements on a larger scale (involving many users) which motivates the search for more robust yet simple modulation schemes that, combined with an appropriate decoding algorithm, show better performance in terms of Packet Error Rate (PER) than classical OFDM systems. This technical criterion translates directly into increased system throughput. Clearly, an attractive property for such a new modulation scheme would be for it to be viewed as a simple extension of OFDM so that it could be implemented in existing standards as a proprietary transmission mode. In this way it could also provide a means for smooth transition to new standards.
In the field of this invention, enhancements have been proposed as a workaround for alleviating an inherent OFDM weakness: when a carrier is subject to a strong channel attenuation, even in absence of noise, the data conveyed is irremediably lost. The classical alternative is to use forward error correction (FEC) coding to spread the information along the carriers, but another strategy has been proposed: to combine the strength of OFDM and CDMA by pre-processing the block of symbols to be transmitted by a unitary spreading matrix W (often chosen to be a Walsh Hadamard transform for its attractive implementation properties) prior to the FFT/IFFT (Fast Fourier Transform/Inverse FFT) modulation.
This redundantless precoder W has the role of uniformly spreading the information to be transmitted on all the carriers so that even if one carrier is unrecoverable, the information transmitted can still be retrieved by decoding of other subbands.
Implementations of such spread OFDM (SOFDM also known as single user OFDM-CDMA with cyclic prefix) modulation techniques require successive interference cancellation (SIC), and many SIC algorithms have been proposed. One of the most well known is ‘V-BLAST’ proposed by Bell Labs for multiple antennas systems in the publication by G. J. Foschini and M. J. Gans, “On Limits of Wireless Communications in a fading Environment when Using Multiple Antennas”, Wireless Personnal Communications 6:311–335, 1998. However, it has been demonstrated (in the publication by P. Loubaton, M. Debbah and M. de Courville, “Spread OFDM Performance with MMSE Equalization”, in International Conference on Acoustics, Speech, and Signal Processing, Salt Lake City, USA, May 2001) that V-BLAST algorithms are not suited for conventional SOFDM systems due to the averaging of the SNRs (signal/noise ratios) at the receiver across the carriers during the despreading step. Moreover, such approaches lead to a tremendous decoding complexity due to the computation of several pseudo inverse matrices.
A need therefore exists for an OFDM communication system and decoding algorithm for use therein wherein the abovementioned disadvantage(s) may be alleviated.